Commercial ships emit almost half as much particulate pollutants into the air globally as the total amount released by the world's cars, according to a new study led by the National Oceanic and Atmospheric Administration and the University of Colorado at Boulder.

The study is the first to provide a global estimate of maritime shipping's total contribution to air particle pollution based on direct emission measurements. The authors estimate ships emit about 1,100 tons of particle pollution globally each year.

Ship pollutants affect both global climate and the health of people living along coastlines, according to the study authors. The findings appear online the week of Feb. 23 in the Journal of Geophysical Research.

"Since more than 70 percent of shipping traffic takes place within 250 miles of the coastline, this is a significant health concern for coastal communities," said lead study author Daniel Lack, a researcher with the NOAA-supported CU Cooperative Institute for Research in Environmental Sciences based at NOAA's Earth System Research Laboratory in Boulder.

Earlier research by one of the study's co-authors, James Corbett of the University of Delaware, linked particle pollution to premature deaths among coastal populations.

Commercial ships emit both particle pollution and carbon dioxide, but they have opposite effects on the climate, said the researchers. The particles have a global cooling effect that is at least five times greater than the global warming effect from the ships' CO2 emissions.

The particles affect both climate and health, said the researchers. CO2 from ships makes up roughly 3 percent of all human-emitted CO2 and almost 30 percent of smog-forming nitrogen oxide gases.

During summer 2006, Lack and colleagues aboard the NOAA ship Ronald H. Brown analyzed the exhaust from over 200 commercial vessels, including cargo ships, tankers and cruise ships in the Gulf of Mexico, Galveston Bay and the Houston Ship Channel. The researchers also examined the chemistry of particles in ship exhaust to understand what makes ships such hefty polluters.

Ships emit sulfates, the same particles associated with diesel-engine cars and trucks and which have resulted in tighter regulations regarding on-road vehicle fuel standards, according to the research team. Sulfate emissions from ships vary with the concentration of sulfur in ship fuel, the authors found.

Globally, fuel sulfur content is capped under the International Convention for the Prevention of Pollution from Ships. As a result of the cap, some ships use "cleaner," low-sulfur fuels, while others continue to use the high-sulfur counterparts.

But sulfates make up just under half of shipping's total particle emissions, according to the NOAA-CU study. Organic pollutants and sooty, black carbon -- which make up the other half of emissions -- are not directly targeted by today's regulations. A 2008 study by Lack's team focused exclusively on soot.

Emissions of non-sulfate particles depend on the operating speed of the engine and the amount of lubricating oil needed to deal with wear and tear from burning less-refined fuels, according to the researchers. "Fortunately, engines burning 'cleaner,' low-sulfur fuels tend to require less complex lubricants," said Corbett. "So the sulfur fuel regulations have the indirect effect of reducing the organic particles emitted."

One surprising result of burning low-sulfur fuels was that while total particle emissions diminish, the time the remaining particles spend in the air appears to increase. It's while they're airborne that particles pose a risk to human health and affect climate, according to the study.

Lack and colleagues found that the organic and black carbon portion of ship exhaust is less likely to form cloud droplets. As a result, the particles remain suspended for longer periods of time before being washed to the ground through precipitation.

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